18 



Acarology 



Grandjean made a study of the nature of the chitin found in the 

 setae of the different groups of mites. He found that certain of the 

 setae of the Trombidiformes and Sarcoptiformes were optically active. 

 The setae of these two groups were readily stained with iodine. The 

 setae of the Mesostigmata and Holothyroidea were found to be opti- 

 cally inactive and resistant to iodine. He designated the active chitin 

 as actinochitin and included the Trombidiformes and Sarcoptiformes 

 in one group, the Actinochitinosi; the Mesostigmata and Holothyroidea 

 are combined as the Anactinochitinosi. Further study is required to 

 determine the significance of a classification based upon the optical 

 activity of the setae. 



Plates: The integument of most acarinids is not uniform in thickness. 



Certain areas are covered by much thicker layers than others and these 

 heavy areas are the plates or shields. They corre- 

 spond to the sclerites of insects but differ from them 

 in that they usually extend over more than a single 

 segment and in some cases may encompass the en- 

 tire idiosoma. Furthermore, in most cases the plates 

 found in the Acariha are not modifications of the 

 primary sclerites but secondarily developed structures. 

 None of the primary sternites remains unmodified. 

 A vestige of the tritosternum remains, however, in the 

 Notostigmata and the Mesostigmata as a peculiar 

 seta-like structure in front of the sternal plate (Figure 

 11). It consists of a basal portion that is usually un- 

 paired from which a pair of pilose lacinae originate. 

 In the Notostigmata the basal portion is paired while 

 in the Uropodina the lacinae are fused basally but 

 may be trifurcate distally. All other plates on the 

 venter of the acarinids are secondary structures and 

 not formed from the primary sternites. 



in a few mites all of the dorsal plates are in reality 

 the modified tergites. They are readily recognizable 

 in the Tarsonemini (Figure 12) and the hysterosomal 

 plates of the Protoplophoridae and Mesoplophoridae 



can be traced back to the primary tergites. In the remainder of the 



mites only the propodosomal plate is a remnant of the primary tergites. 



It is formed by a fusion of the tergites of the segments of the first two 



pairs of legs; that is, segments iv and v. 



Figure 11 The 



tritosternum of 

 Haemolaelaps 

 traiibi (Strandt- 

 mann), 1948. 

 (After Strandt- 

 mann 1948) 



